A microstrip antenna called a patch antenna where one side surface of a dielectric substrate is covered with a ground electrode and the other side surface of the dielectric substrate is provided with a rectangular or circular radiation electrode, has been known. The patch antenna may be made thin and has a high gain, and thus is being used in various applications.
In the patch antenna, there is suggested a technology of forming a cutout in a radiation electrode to adjust a property of the antenna.
For example, eight slit-like cutouts are formed in a radiation conductor formed in a square shape of a planar antenna. These slit-like cutouts are formed in a parallel direction with respect to an arbitrary side from the respective sides of the radiation conductor and at positions where the radiation conductor has the same shape even if rotated by 90°. Accordingly, an impedance change with respect to a distance change from an origin to a feeding point becomes relatively small so that an impedance matching is easily performed with the planar antenna, and at the same time, the planar antenna has a wide bandwidth.
For example, a radiation electrode is formed to have an external shape having cutout portions at four positions of a patch antenna, and each cutout portion is formed at a position facing a substantial center of four sides of a dielectric substrate. Then, since the patch antenna has the cutout portions, a downward radiation is suppressed to increase a gain in a zenith direction.
For example, a leg side extends from a cutout portion formed at the center of each side of a substantially squared radiation conductor plate of a circularly polarized wave antenna. A gap between opposed leg sides is set to be longer than a gap between opposed leg sides at the other side by a predetermined length. It is set that a diagonal line on which a feeding pin is present has an angle of 45° with respect to a straight line A having one side opposed leg sides present at both ends, and a straight line B having the other side opposed leg sides present at both ends. Accordingly, since a prescribed difference occurs in a resonance length between a resonance mode along the straight line A and a resonance mode along the straight line B, the antenna operates as a circularly polarized wave antenna.
In a patch antenna in which a radiation electrode is formed in a substantially rectangular shape, the patch antenna resonates with respect to radio waves having a polarization plane along a long side direction of the radiation electrode and also having a wavelength twice the length of the long side. Likewise, the patch antenna resonates with respect to radio waves having a polarization plane along a short side direction of the radiation electrode and also having a wavelength twice the length of the short side. Therefore, the patch antenna may radiate or receive radio waves having a polarization plane along a long side of the radiation electrode and also having a wavelength twice the length of the long side, and radio waves having a polarization plane along a short side of the radiation electrode and also having a wavelength twice the length of the short side.
Meanwhile, in some applications, such as radar, a patch antenna is required to radiate or receive radio waves having a polarization plane in a specific direction, and to suppress radiation or reception of radio waves having a polarization plane in the other direction. In such a case, in each technology described above, since a slit or a cutout portion is formed at each of four sides of the radiation electrode, it is difficult to radiate or receive radio waves having a polarization plane in a specific direction, and difficult to suppress radiation or reception of radio waves having a polarization planes in the other direction.
The followings are reference documents.                [Document 1] Japanese Laid-Open Patent Publication No. 5-304413,        [Document 2] Japanese Laid-Open Patent Publication No. 2005-203873, and        [Document 3] Japanese Laid-Open Patent Publication No. 2005-252585.        